Research On Model Predictive Control Strategy For Fault-tolerant Three-phase Four-switch Rectifiers

Three-phase six-switch rectifiers are widely used in electric vehicles,flexible AC transmission systems,photovoltaic grid-connected systems,energy storage systems,etc.However,since the power switching devices are in the process of high-frequency switching on or off,it may easily cause the problem of switching devices are in short circuit or open circuit fault.It should be noted that the three-phase six-switch rectifier has no fault-tolerant performance.Once the rectifier fault occurs,it will pose a major threat to the safe and stable operation of the system.Therefore,it is of great practical significance to study the converter with fault-tolerant performance and the corresponding control strategy.The fault-tolerant three-phase four-switch rectifier is studied by an increasing number of scholars because of its simple structure and low cost.The scheme,the fault-tolerant three-phase four-switch rectifier selected as the research object will be studied in depth.In the environment of continuous development of science and technology,most widespread control strategies of converter have been deeply studied,such as PI feedforward decoupling control,fuzzy control,hysteresis control,sliding mode control,deadbeat control,etc.In recent years,a Finite Control Set Model Predictive Control(FCS-MPC)with superior control performance have attracted the curiosity of a wide range of scholars.Compared with the traditional control strategy,FCS-MPC is simple and easy to implement,which can improve the steady state performance of the system and reduce the grid-side current harmonic content.It will be studied in this paper that the FCS-MPC strategy under the topological structures of the three-phase six-switch rectifier and the three-phase four-switch rectifier to improve the steady-state performance of the system as far as possible and to ensure the stable operation of the system without any interruption.The main contents of this paper are as follows:(1)In order to reduce the computation of traditional model predictive control and further improve the steady-state performance of the system,an improve multi-vector model predictive control strategy based simplified calculation for three-phase six-switch rectifier is proposed.Three voltage vectors(two non-zero voltage vectors and one zero voltage vector)are selected for control system in each sampling period,and each voltage vector is optimally selected.Meanwhile,the sector division idea is adopted to simplify the calculation.In addition,by calculating action time of three selected optimized voltage vectors,the required pulses are obtained.The simulation results show that compared with the traditional model predictive control,the proposed strategy can better reduce the power ripples of the system and improve the quality of the grid side current.Specifically,it achieve a constant switching frequency with the reduction of calculation amount.(2)To cope with the DC-link capacitor voltage unbalancing for the three-phase four-switch rectifier,a FCS-MPC for the three-phase four-switch rectifier is proposed.By adding the target term of suppress the capacitor voltage fluctuation to the cost function,the multi-objective variable cooperative control is realized.The simulation results show that the proposed strategy can make the system run stably and ensure the capacitor voltage balancing.(3)In order to solve the problem of DC-link capacitor voltage imbalance of three-phase four-switch rectifier and improve the uninterrupted and stable operation performance of the system,a multi-vector model predictive control for three-phase four-switch rectifier is proposed,meanwhile power compensation strategy is adopted to complete DC-link capacitor voltage balancing control.The voltage-vector combination of collinear and opposite directions and unequal values is used to solve the zero-free voltage vector problem.In the multi-vector model predictive control,three voltage vectors(all non-zero voltage vectors)are selected for each sampling period,so that the action range of the equivalent composite vector is expanded,and the control precision is improved.The simulation results show that compared with the traditional model predictive control,the proposed strategy enables the system to possess favorable capacitor voltage balancing characteristics,meanwhile can batter reduce the system power ripples and the harmonic content of the grid side current.In addition,the constant switching frequency and the regular harmonic distribution law should be noted.